add link to begginer tips. didn't add directly to readme since a lot …

…of the code might change soon. (#10)

README.md

@@ -73,6 +73,7 @@ python sim/humanoid_gym/train.py --task=stompy_ppo --num_envs=4096 --headless
 ```bash
 python sim/humanoid_gym/play.py --task legs_ppo --sim_device cpu
 ```
+See [this doc](https://docs.google.com/document/d/1YZzBqIXO7oq7vIKu-BZr5ssNsi3nKtxpRPnfSnTXojA/edit?usp=sharing) for more beginner tips.
 
 ### Errors
 After cloning Isaac Gym, sometimes the bindings mysteriously disappear.